1. Technical Field
This disclosure relates to a developing device for developing an electrostatic image with a one component developer (i.e., a toner). In addition, this disclosure also relates to a one component developer for the developing device, and an image forming method, an image forming apparatus and a process cartridge using the developing device.
2. Discussion of the Related Art
Recently, a need exists for a low end laser beam printer which is low-cost and compact and which can perform high speed printing. In order to miniaturize a laser beam printer, it is essential to use a fixing device, which does not use an oil application device configured to apply an oil to the fixing member thereof and which uses two rollers (such as combinations of a fixing roller (heat roller) and a pressure roller) for fixing toner images. In this case, since an oil application device is not used, it is essential to include a release agent (such as waxes) in the toner used for the developing device. However, toner including a release agent therein has large adhesiveness and cohesiveness, and therefore the torque has to be increased to well agitate the toner (or the developer) in the developing device.
In addition, in order to perform high speed image formation, it is preferable to use a tandem image forming apparatus in which four image forming units each including a photoreceptor, a developing device, and other devices are serially arranged. In such a tandem image forming apparatus, the four image forming units are arranged in a horizontal direction to prevent increase in height of the image forming apparatus. Therefore, it is necessary to use a vertical developing device for such image forming units.
When a vertical developing device is used, all the weight of the toner in the container of the developing device is applied to the developing roller, and therefore the toner tends to stay around a developer layer forming blade, which scrapes the toner to form a thin layer of the toner on the developing roller. In this case, a toner adhesion problem in that the toner is adhered to the developer layer forming blade, resulting in formation of low density images or uneven density images occurs. Alternatively, an uneven density image problem in that when images with high image area proportion are formed, the resultant images have uneven image density because the toner cannot be well supplied to the photoreceptor occurs.
In attempting to prevent occurrence of such problems, a technique in that a polymerization toner which has a spherical form and has a relatively weak adhesiveness is used, and a technique in that a release agent is included in a polymerization toner have been proposed. However, the techniques have the following drawbacks:
(1) The toners have high costs because expensive materials have to be used for the reaction of forming the binder resin of the toner;
(2) The costs of the facilities for producing such polymerization toners are high because, for example, a facility for removing and collecting volatile materials is necessary; and
(3) Since a large amount of water is used for manufacturing such polymerization toners, severe location requirements are set for manufacturing the toners.
Therefore, it is hard to use such toners for low end laser beam printers.
Published unexamined Japanese patent application No. 2003-084563 discloses a toner layer forming member which is formed of a plate, wherein the plate is bent at two portions of the tip thereof such that the ridge lines of the bending portions are parallel to each other and the ridge lines are present on the opposite sides of the plate.
However, even when such a toner layer forming member is used for a low end printer, it is hard to use a polymerization toner (which has a spherical form) for the low end printer.
Because of these reasons, a need exists for a vertical developing device which does not cause the toner adhesion problem and uneven density image problem.